X-ray devices used in the medical field contain an X-ray tube which typically includes a stationary cathode which is heated to emit electrons, a rotating anode having a target surface attached to a cap and generally facing the cathode, and a surrounding glass and/or metal stationary frame containing a vacuum and having an X-ray-transparent window. Some emitted electrons strike the target surface and produce X-rays, and some of the X-rays exit the frame as an X-ray beam through the X-ray-transparent window. Load-carrying bearings are located inside the vacuum between the frame and a stem attached to the cap. The bearings are lubricated by special dry lubricants or by special liquid gallium lubricants. The lubricants tend, in the vacuum, to migrate away from the bearings which will cause increased bearing friction (and which can also cause high voltage instability) which shortens the operating life of the X-ray tube. The target surface is heated by the impinging electrons. The heat is dissipated by thermal radiation from the target surface through the vacuum to the frame and by thermal conduction from the target surface to the cap to the stem to the bearings to the frame. The heated frame is typically cooled by a liquid coolant, such as oil or water, located between the frame and a surrounding casing which has its own X-ray transparent window. The limited cooling arrangement of the tube design restricts the X-ray power output.
What is needed is an improved X-ray tube design having improved lubrication of the bearings and improved cooling of the anode stem.